Microlens-assisted brightness enhancement in reflective image displays

ABSTRACT

A reflective display having a light reflector ( 178 ) incorporating a microlens array ( 180 ) and a reflective surface ( 190 ). The microlenses redirect light onto reflective regions ( 194 ) of the surface. An electrode on the surface has a plurality of annular segments ( 192 ), each segment being aligned with a microlens. An electrophoresis medium ( 204 ) is contained between the array and the reflective surface. Light absorptive particles are suspended in the medium. An electrical potential source applies an electrical potential across the medium. In the reflective state, the particles are attracted to the electrode segments, leaving the reflective regions substantially unobstructed, and permitting reflection of light by the reflective regions. In the absorptive, non-reflective state the particles are attracted to and distributed across an inward surface ( 206 ) of the microlens array. Light which passes through the array is absorbed by the particles at the inward surface of the microlens array.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional patentapplication Ser. No. 60/759,772 filed 17 Jan. 2006.

TECHNICAL FIELD

This application pertains to brightness enhancement of reflective imagedisplays of the type described in U.S. Pat. Nos. 5,999,307; 6,064,784;6,215,920; 6,865,011; 6,885,496 and 6,891,658; in United States PatentApplication Publication No. 2006-0209418-A1; and in International PatentPublication No. WO 2006/108285 all of which are incorporated herein byreference.

BACKGROUND

FIG. 1A depicts a portion of a prior art reflective (i.e. front-lit)image display 10 in which total internal reflection (TIR) iselectrophoretically modulated as described in U.S. Pat. Nos. 6,885,496and 6,891,658. Display 10 includes a transparent outward sheet 12 formedby partially embedding a large plurality of high refractive index (e.g.η₁>˜1.90) transparent spherical or approximately spherical beads 14 inthe inward surface of a high refractive index (e.g. η₂>˜1.75) polymericmaterial 16 having a flat outward viewing surface 17 which viewer Vobserves through an angular range of viewing directions Y. The “inward”and “outward” directions are indicated by double-headed arrow Z. Beads14 are packed closely together to form an inwardly projecting monolayer18 having a thickness approximately equal to the diameter of one ofbeads 14. Ideally, each one of beads 14 touches all of the beadsimmediately adjacent to that one bead. Minimal interstitial gaps(ideally, no gaps) remain between adjacent beads.

An electrophoresis medium 20 is maintained adjacent the portions ofbeads 14 which protrude inwardly from material 16 by containment ofmedium 20 within a reservoir 22 defined by lower sheet 24. An inert, lowrefractive index (i.e. less than about 1.35), low viscosity,electrically insulating liquid such as Fluorinert™ perfluorinatedhydrocarbon liquid (η₃˜1.27) available from 3M, St. Paul, Minn. is asuitable electrophoresis medium. Other liquids, or water can also beused as electrophoresis medium 20. A bead:liquid TIR interface is thusformed. Medium 20 contains a finely dispersed suspension of lightscattering and/or absorptive particles 26 such as pigments, dyed orotherwise scattering/absorptive silica or latex particles, etc. Sheet24's optical characteristics are relatively unimportant: sheet 24 needonly form a reservoir for containment of electrophoresis medium 20 andparticles 26, and serve as a support for backplane electrode 48.

As is well known, the TIR interface between two media having differentrefractive indices is characterized by a critical angle θ_(c). Lightrays incident upon the interface at angles less than θ_(c) aretransmitted through the interface. Light rays incident upon theinterface at angles greater than θ_(c) undergo TIR at the interface. Asmall critical angle is preferred at the TIR interface since thisaffords a large range of angles over which TIR may occur.

In the absence of electrophoretic activity, as is illustrated to theright of dashed line 28 in FIG. 1A, a substantial fraction of the lightrays passing through sheet 12 and beads 14 undergoes TIR at the inwardside of beads 14. For example, incident light rays 30, 32 are refractedthrough material 16 and beads 14. The rays undergo TIR two or more timesat the bead:liquid TIR interface, as indicated at points 34, 36 in thecase of ray 30; and indicated at points 38, 40 in the case of ray 32.The totally internally reflected rays are then refracted back throughbeads 14 and material 16 and emerge as rays 42, 44 respectively,achieving a “white” appearance in each reflection region or pixel.

A voltage can be applied across medium 20 via electrodes 46, 48 (shownas dashed lines) which can for example be applied by vapour-depositionto the inwardly protruding surface portion of beads 14 and to theoutward surface of sheet 24. Electrode 46 is transparent andsubstantially thin to minimize its interference with light rays at thebead:liquid TIR interface. Backplane electrode 48 need not betransparent. If electrophoresis medium 20 is activated by actuatingvoltage source 50 to apply a voltage between electrodes 46, 48 asillustrated to the left of dashed line 28, suspended particles 26 areelectrophoretically moved into the region where the evanescent wave isrelatively intense (i.e. within 0.25 micron of the inward surfaces ofinwardly protruding beads 14, or closer). When electrophoretically movedas aforesaid, particles 26 scatter or absorb light, thus frustrating TIRby modifying the imaginary and possibly the real component of theeffective refractive index at the bead:liquid TIR interface. This isillustrated by light rays 52, 54 which are scattered and/or absorbed asthey strike particles 26 inside the thin (˜0.5 μm) evanescent waveregion at the bead:liquid TIR interface, as indicated at 56, 58respectively, thus achieving a “dark” appearance in each TIR-frustratednon-reflective absorption region or pixel. Particles 26 need only bemoved outside the thin evanescent wave region, by suitably actuatingvoltage source 50, in order to restore the TIR capability of thebead:liquid TIR interface and convert each “dark” non-reflectiveabsorption region or pixel to a “white” reflection region or pixel.

As described above, the net optical characteristics of outward sheet 12can be controlled by controlling the voltage applied across medium 20via electrodes 46, 48. The electrodes can be segmented to control theelectrophoretic activation of medium 20 across separate regions orpixels of sheet 12, thus forming an image.

FIG. 2 depicts, in enlarged cross-section, an inward hemispherical or“hemi-bead” portion 60 of one of spherical beads 14. Hemi-bead 60 has anormalized radius r=1 and a refractive index η₁. A light ray 62perpendicularly incident (through material 16) on hemi-bead 60 at aradial distance a from hemi-bead 60's centre C encounters the inwardsurface of hemi-bead 60 at an angle θ₁ relative to radial axis 66. Forpurposes of this theoretically ideal discussion, it is assumed thatmaterial 16 has the same refractive index as hemi-bead 60 (i.e. η₁=η₂),so ray 62 passes from material 16 into hemi-bead 60 without refraction.Ray 62 is refracted at the inward surface of hemi-bead 60 and passesinto electrophoretic medium 20 as ray 64 at an angle θ₂ relative toradial axis 66.

Now consider incident light ray 68 which is perpendicularly incident(through material 16) on hemi-bead 60 at a distance

$a_{c} = \frac{\eta_{3}}{\eta_{1}}$

from hemi-bead 60's centre C. Ray 68 encounters the inward surface ofhemi-bead 60 at the critical angle θ_(c) (relative to radial axis 70),the minimum required angle for TIR to occur. Ray 68 is accordinglytotally internally reflected, as ray 72, which again encounters theinward surface of hemi-bead 60 at the critical angle θ_(c). Ray 72 isaccordingly totally internally reflected, as ray 74, which alsoencounters the inward surface of hemi-bead 60 at the critical angleθ_(c). Ray 74 is accordingly totally internally reflected, as ray 76,which passes perpendicularly through hemi-bead 60 into the embeddedportion of bead 14 and into material 16. Ray 68 is thus reflected backas ray 76 in a direction approximately opposite that of incident ray 68.

All light rays which are incident on hemi-bead 60 at distances a≧a_(c)from hemi-bead 60's centre C are reflected back (but not exactlyretro-reflected) toward the light source; which means that thereflection is enhanced when the light source is overhead and slightlybehind the viewer, and that the reflected light has a diffusecharacteristic giving it a white appearance, which is desirable inreflective display applications. FIGS. 3A, 3B and 3C depict three ofhemi-bead 60's reflection modes. These and other modes coexist, but itis useful to discuss each mode separately.

In FIG. 3A, light rays incident within a range of distances a_(c)<a≦a₁undergo TIR twice (the 2-TIR mode) and the reflected rays diverge withina comparatively wide arc φ₁ centred on a direction opposite to thedirection of the incident light rays. In FIG. 3B, light rays incidentwithin a range of distances a₁<a≦a₂ undergo TIR three times (the 3-TIRmode) and the reflected rays diverge within a narrower arc φ₂<φ₁ whichis again centred on a direction opposite to the direction of theincident light rays. In FIG. 3C, light rays incident within a range ofdistances a₂<a≦a₃ undergo TIR four times (the 4-TIR mode) and thereflected rays diverge within a still narrower arc φ₃<φ₂ also centred ona direction opposite to the direction of the incident light rays.Hemi-bead 60 thus has a “semi-retro-reflective,” partially diffusereflection characteristic, causing display 10 to have a diffuseappearance akin to that of paper.

Display 10 has relatively high apparent brightness, comparable to thatof paper, when the dominant source of illumination is behind the viewer,within a small angular range. This is illustrated in FIG. 1B whichdepicts the wide angular range α over which viewer V is able to viewdisplay 10, and the angle β which is the angular deviation ofillumination source S relative to the location of viewer V. Display's10's high apparent brightness is maintained as long as β is not toolarge. At normal incidence, the reflectance R of hemi-bead 60 (i.e. thefraction of light rays incident on hemi-bead 60 that reflect by TIR) isgiven by equation (1):

$\begin{matrix}{R = {1 - \left( \frac{\eta_{3}}{\eta_{1}} \right)^{2}}} & (1)\end{matrix}$

where η₁ is the refractive index of hemi-bead 60 and η₃ is therefractive index of the medium adjacent the surface of hemi-bead 60 atwhich TIR occurs. Thus, if hemi-bead 60 is formed of a lower refractiveindex material such as polycarbonate (η₁˜1.59) and if the adjacentmedium is Fluorinert (η₃˜1.27), a reflectance R of about 36% isattained, whereas if hemi-bead 60 is formed of a high refractive indexnano-composite material (η₁˜1.92) a reflectance R of about 56% isattained. When illumination source S (FIG. 1B) is positioned behindviewer V's head, the apparent brightness of display 10 is furtherenhanced by the aforementioned semi-retro-reflective characteristic.

As shown in FIGS. 4A-4G, hemi-bead 60's reflectance is maintained over abroad range of incidence angles, thus enhancing display 10's wideangular viewing characteristic and its apparent brightness. For example,FIG. 4A shows hemi-bead 60 as seen from perpendicular incidence—that is,from an incidence angle offset 0° from the perpendicular. In this case,the portion 80 of hemi-bead 60 for which a≧a_(c) appears as an annulus.The annulus is depicted as white, corresponding to the fact that this isthe region of hemi-bead 60 which reflects incident light rays by TIR, asaforesaid. The annulus surrounds a circular region 82 which is depictedas dark, corresponding to the fact that this is the non-reflectiveregion of hemi-bead 60 within which incident rays are absorbed and donot undergo TIR. FIGS. 4B-4G show hemi-bead 60 as seen from incidentangles which are respectively offset 15°, 30°, 45°, 60°, 75° and 90°from the perpendicular. Comparison of FIGS. 4B-4G with FIG. 4A revealsthat the observed area of reflective portion 80 of hemi-bead 60 forwhich a≧a_(c) decreases only gradually as the incidence angle increases.Even at near glancing incidence angles (e.g. FIG. 4F) an observer willstill see a substantial part of reflective portion 80, thus givingdisplay 10 a wide angular viewing range over which high apparentbrightness is maintained.

An estimate of the reflectance of an array of hemispheres correspondingto the inward “hemi-bead” portions of each one of spherical beads 14depicted in FIG. 1A can be obtained by multiplying the reflectance of anindividual hemi-bead by the hemi-beads' packing efficiency coefficientƒ. Calculation of the packing efficiency coefficient ƒ of a closelypacked structure involves application of straightforward geometrytechniques which are well known to persons skilled in the art. Thehexagonal closest packed (HCP) structure depicted in FIG. 5 yields apacking efficiency ƒ∞π/(6·tan 30°)˜90.7% assuming beads 14 are ofuniform size.

Although the HCP structure yields the highest packing density forhemispheres, it is not necessary to pack the hemi-beads in a regulararrangement, nor is it necessary that the hemi-beads be of uniform size.A random distribution of non-uniform size hemi-beads having diameterswithin a range of about 1-50 μm has a packing density of approximately80%, and has an optical appearance substantially similar to that of anHCP arrangement of uniform size hemi-beads. For some reflective displayapplications, such a randomly distributed arrangement may be morepractical to manufacture, and for this reason, somewhat reducedreflectance due to less dense packing may be acceptable. However, forsimplicity, the following description focuses on the FIG. 5 HCParrangement of uniform size hemi-beads, and assumes the use of materialswhich yield a refractive index ratio η₁/η₃=1.5. These factors are not tobe considered as limiting the scope of this disclosure.

As previously explained in relation to FIG. 2, a substantial portion oflight rays which are perpendicularly incident on the flat outward faceof hemi-bead 60 at distances a<a_(c) from hemi-bead 60's centre C do notundergo TIR and are therefore not reflected by hemi-bead 60. Instead, asubstantial portion of such light rays are scattered and/or absorbed byprior art display 10, yielding a dark non-reflective circular region 82(FIGS. 4A-4G) on hemi-bead 60. FIG. 5 depicts a plurality of these darknon-reflective regions 82, each of which is surrounded by a reflectiveannular region 80, as previously explained.

Hemi-bead 60's average surface reflectance, R, is determined by theratio of the area of reflective annulus 80 to the total area comprisingreflective annulus 80 and dark circular region 82. That ratio is in turndetermined by the ratio of the refractive index, η₁, of hemi-bead 60 tothe refractive index, Θ₃, of the medium adjacent the surface ofhemi-bead 60 at which TIR occurs, in accordance with Equation (1). It isthus apparent that the average surface reflectance, R, increases withthe ratio of the refractive index η₁, of hemi-bead 60 to that of theadjacent medium η₃. For example, the average surface reflectance, R, ofa hemispherical water drop (η₁˜1.33) in air (η₃˜1.0) is about 43%; theaverage surface reflectance, R, of a glass hemisphere (η₁˜1.5) in air isabout 55%; and the average surface reflectance, R, of a diamondhemi-sphere (η₁˜2.4) in air exceeds 82%.

Although it may be convenient to fabricate display 10 using spherically(or hemispherically) shaped beads as aforesaid, even if spherical (orhemispherical) beads 14 are packed together as closely as possiblewithin monolayer 18 (FIG. 1A), interstitial gaps 84 (FIG. 5) unavoidablyremain between adjacent beads. Light rays incident upon any of gaps 84are “lost” in the sense that they pass directly into electrophoreticmedium 20, producing undesirable dark spots on viewing surface 17. Whilethese spots are invisibly small, and therefore do not detract fromdisplay 10's appearance, they do detract from viewing surface 17's netaverage surface reflectance, R.

The above-described “semi-retro-reflective” characteristic is importantin a reflective display because, under typical viewing conditions wherelight source S is located above and behind viewer V, a substantialfraction of the reflected light is returned toward viewer V. Thisresults in an apparent reflectance which exceeds the value

$R = {1 - \left( \frac{\eta_{3}}{\eta_{1}} \right)^{2}}$

by a “semi-retro-reflective enhancement factor” of about 1.5 (see “AHigh Reflectance, Wide Viewing Angle Reflective Display Using TotalInternal Reflection in Micro-Hemispheres,” Mossman, M. A. et al.,Society for Information Display, 23rd International Display ResearchConference, pages 233-236, Sep. 15-18, 2003, Phoenix, Ariz.). Forexample, in a system where the refractive index ratio η₁/η₃=1.5, theaverage surface reflectance, R, of 55% determined in accordance withEquation (1) is enhanced to approximately 85% under thesemi-retro-reflective viewing conditions described above.

Individual hemi-beads 60 can be invisibly small, within the range of2-50 μm in diameter, and as shown in FIG. 5 they can be packed into anarray to create a display surface that appears highly reflective due tothe large plurality of tiny, adjacent, reflective annular regions 80. Inthese regions 80, where TIR can occur, particles 26 (FIG. 1A) do notimpede the reflection of incident light when they are not in contactwith the inward, hemispherical portions of beads 14. However, in regions82 and 84, where TIR does not occur, particles 26 may absorb incidentlight rays—even if particles 26 are moved outside the evanescent waveregion so that they are not in optical contact with the inward,hemispherical portions of beads 14. The refractive index ratio η₁/η₃ canbe increased in order to increase the size of each reflective annularregion 80 and thus reduce such absorption losses. Non-reflective regions82, 84 cumulatively reduce display 10's overall surface reflectance, R.Since display 10 is a reflective display, it is clearly desirable tominimize such reduction.

Disregarding the aforementioned semi-retro-reflective enhancementfactor, a system having a refractive index ratio η₁/η₃=1.5 has anaverage surface reflectance, R, of 55%, as previously explained. Giventhe HCP arrangement's aforementioned packing efficiency of about 91%,the system's overall average surface reflectance is 91% of 55% or about50%, implying a loss of about 50%. 41% of this loss is due to lightabsorption in circular non-reflective regions 82; the remaining 9% ofthis loss is due to light absorption in interstitial non-reflective gaps84. Display 10's reflectance can be increased by decreasing suchabsorptive losses through the use of materials having specific selectedrefractive index values, optical microstructures or patterned surfacesplaced on the outward or inward side(s) of monolayer 18 (FIG. 1A).

For example, since display 10's maximum surface reflectance isdetermined by the ratio of the refractive index values of hemi-bead 60and electrophoretic medium 20, the reflectance can be increased bysubstituting air (refractive index=1.0) as electrophoretic medium 20instead of a low refractive index liquid (refractive index less than1.35).

Display 10's surface reflectance can be increased, as described below,thereby further improving the appearance of the display.

The foregoing examples of the related art and limitations relatedthereto are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIG. 1A is a greatly enlarged, not to scale, fragmented cross-sectionalside elevation view of a portion of a prior art reflective image displayin which TIR is electrophoretically modulated.

FIG. 1B schematically illustrates the wide angle viewing range α of theFIG. 1A display, and the angular range β of the illumination source.

FIG. 2 is a cross-sectional side elevation view, on a greatly enlargedscale, of a hemispherical (“hemi-bead”) portion of one of the sphericalbeads of the FIG. 1A apparatus.

FIGS. 3A, 3B and 3C depict semi-retro-reflection of light raysperpendicularly incident on the FIG. 2 hemi-bead at increasing off-axisdistances at which the incident rays undergo TIR two, three and fourtimes respectively.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F and 4G depict the FIG. 2 hemi-bead, as seenfrom viewing angles which are offset 0°, 15°, 30°, 45°, 60°, 75° and 90°respectively from the perpendicular.

FIG. 5 is a top plan (i.e. as seen from a viewing angle offset 0° fromthe perpendicular) cross-sectional view of a portion of the FIG. 1display, showing the spherical beads arranged in a hexagonal closestpacked (HCP) structure.

FIG. 6A is a cross-sectional side elevation view, on a greatly enlargedscale, of a portion of a reflector incorporating a microlens arrayoptically coupled to a reflective surface. FIG. 6B schematically depictsfocusing of light rays by the FIG. 6A structure.

FIG. 7 is a cross-sectional side elevation view, on a greatly enlargedscale, of a portion of another reflector incorporating a microlens arrayoptically coupled to a reflective surface.

FIGS. 8A and 8B respectively schematically depict electrophoreticmodulation of the FIG. 6A structure, with FIG. 8A depicting thereflective state and FIG. 8B depicting the absorptive, non-reflectivestate.

FIGS. 9A and 9B respectively schematically depict modulation of the FIG.6A structure by electro-deformation of a liquid medium, with FIG. 9Adepicting the reflective state and FIG. 9B depicting the absorptive,non-reflective state.

FIGS. 10A and 10B respectively schematically depict electrophoreticmodulation of the FIG. 7 structure, with FIG. 10A depicting thereflective state and FIG. 10B depicting the absorptive, non-reflectivestate.

FIGS. 11A and 11B respectively schematically depict modulation of theFIG. 7 structure by electro-deformation of a liquid medium, with FIG.11A depicting the reflective state and FIG. 11B depicting theabsorptive, non-reflective state.

DESCRIPTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding to persons skilled in theart. However, well known elements may not have been shown or describedin detail to avoid unnecessarily obscuring the disclosure. Accordingly,the description and drawings are to be regarded in an illustrative,rather than a restrictive, sense.

In general, it is known that a reflective display can be formed byproviding a spatially variable small scale pattern on an opticalsurface. Light rays are substantially non-absorbed by a first areafraction, ƒ_(w), of the pattern, and substantially absorbed by a second,complementary, area fraction, ƒ_(b)=1−ƒ_(w), OF the pattern. The valueof ƒ_(w) can be modified on a small size scale to vary the pattern's neteffective reflectance as a function of macroscopic position on theoptical surface in order to produce a graphic image. This is typicallyachieved by placing a highly reflective surface immediately behind theoptical surface—in which case the net effective reflectance isapproximately equal to ƒ_(w).

Reflective displays based on this concept are for example disclosed byHayes et al. in “Video-Speed Electronic Paper Based on Electrowetting,”Nature, Vol. 425, pp. 383-385, 25 Sep. 2003; Kishi et al. in“Development of In-Plane EPD,” pp. 24-27, Proceedings of the Society forInformation Display Symposium 2000; and Swanson et al. in “HighPerformance Electrophoretic Displays,” pp. 29-31 Proceedings of theSociety for Information Display Symposium 2000. However, the efficacy ofsuch displays is limited by the value of ƒ_(w), which cannot exceed 1,whereas in most practical displays it is difficult to achieve a value ofƒ_(w) exceeding about 0.6. This is problematic because a net effectivereflectance of about 2 is required to implement red-green-blue (RGB)filtering in a colour reflective display having a brightness comparableto that of coloured ink on white paper. (It is possible, in principle,to attain a net effective reflectance of about 2 in a reflectivedisplay, because the incident light is usually derived from a restrictedrange of directions. A selectively reflective surface can accordinglyproduce an enhanced effective reflectance value.)

FIG. 6A depicts a portion of a reflector 178 positionable on therearward side of a reflective image display. Reflector 178 incorporatesmicrolens array 180 and reflective optical surface 190. Microlens array180 in turn incorporates microlenses 182, 184, 186, 188 which arepositioned in front of (i.e. on an outward side of) reflective surface190 and which protrude outwardly away from reflective surface 190. Asshown in FIG. 6B, an optically masked (i.e. invisible) annular region80A encircling a non-masked (i.e. visible) circular region 82A isassociated with each microlens. As explained below, annular regions 80Aare not visible when viewed from the normal viewing direction, due tothe refraction caused by the microlens; only circular regions 82A arevisible when viewed from the normal viewing direction.

The periodicity of microlenses 182-188 matches that of a lightabsorptive pattern applied to surface 190. Specifically, an annularpattern segment 192 is concentrically aligned beneath each one ofmicrolenses 182-188, as shown in FIGS. 6A and 6B. The opticalcharacteristics of segments 192 are relatively unimportant, but segments192 may be electrically conductive to facilitate controllable attractiontoward segments 192 of a light absorber, as explained below. Eachsegment 192 has approximately the same size and shape as, and isconcentrically aligned with, the optically masked annular region 80A ofthe adjacent microlens. Each segment 192 encloses a circular, reflectiveportion 194 of surface 190. Each portion 194 has approximately the samesize and shape as, and is aligned with, the non-optically maskedcircular region 82A of the adjacent microlens.

As shown schematically in FIG. 6B, each microlens focuses (i.e.converges) incident light rays 195 (i.e. light rays which arrive fromapproximately the typical viewing direction) toward a focal point 197beneath the microlens. Surface 190 is positioned between microlens array180 and focal point 197, and is spaced an appropriate distance d₁beneath microlens array 180, such that light rays 195 are focused by themicrolens onto the circular, reflective portion 194 of surface 190underlying the microlens. Distance d₁ is selected such thatsubstantially all light rays which pass through any one of microlenses182-188 are focused onto the circular, reflective portion 194 beneaththat microlens, and such that substantially no light rays which passthrough any one of microlenses 182-188 reach the annular pattern segment192 beneath that microlens. Without microlens array 180, surface 190would reflect only a fraction of the incident light rays, correspondingto the ratio of the total areas of reflective portions 194 and surface190 respectively. However, since microlens array 180 focusessubstantially all incident light rays onto surface 190's circular,reflective portions 194, substantially all incident light rays arereflected, thus enhancing the apparent brightness of surface 190. Sincerelatively few light rays reach annular pattern segments 192, they areeffectively hidden from view (i.e. optically masked, as aforesaid).

More particularly, as shown in FIG. 6A, incident light ray 196 isfocused by microlens 184 onto the circular, reflective portion 194beneath microlens 184, which reflects the ray back through microlens 184such that the ray emerges as reflected ray 198. Incident light ray 200is similarly focused by microlens 184 onto the circular, reflectiveportion 194 beneath microlens 184, which again reflects the ray backthrough microlens 184 such that the ray emerges as reflected ray 202.

It can thus be seen that light rays are converged by microlenses 182-188away from annular pattern segments 192 onto surface 190's circular,reflective portions 194. Although only a fractional portion, ƒ_(w), ofsurface 190 (i.e. the circular, reflective portions 194 thereof whichare devoid of annular pattern segments 192) is reflective, essentiallyall light rays of interest are directed toward that fractional portion,increasing the effective value of ƒ_(w) to approximately 1.

Reflective surface 190 may be diffusely reflective, or specularlyreflective, or semi-retro-reflective, or retro-reflective. The magnitudeof the brightness enhancement of surface 190 will depend on itsreflectivity characteristic. For example, the brightness of surface 190will be enhanced if surface 190 is diffusely reflective, but themagnitude of the brightness enhancement will be relatively small sinceonly a fraction of the diffusely reflected light rays will be reflectedwithin the angular range of viewing directions Y (FIG. 6A) through whichviewer V observes reflector 178. The magnitude of the brightnessenhancement will be greater if surface 190 is specularly reflective, butthat magnitude will depend on the viewing angle. The magnitude of thebrightness enhancement will be even greater if surface 190 issemi-retro-reflective, since light rays will be semi-retro-reflected ina manner which is relatively independent of the viewing angle. Themagnitude of the brightness enhancement will be greater still if surface190 is retro-reflective, since retro-reflected light rays return to theviewer essentially independently of the viewing angle.Semi-retro-reflective and specularly reflective characteristics aredesirable because they facilitate significant brightness enhancement,without highly collimating the reflected light which accordingly appearswhite.

FIGS. 6A and 6B depict only one possible configuration for microlensarray 180. For example, although they are depicted as hemispherical,microlenses 182-188 need not be hemispherical. Any microlens shapecapable of focusing a substantial fraction of incident light rays onto areflective region beneath the microlens will suffice. Persons skilled inthe art will understand that there are many different appropriatemicrolens configurations—the configuration shown in FIGS. 6A and 6Bshould not be considered to be limiting.

The embodiment of FIGS. 6A and 6B selectively redirects light rays backinto the direction from which they came, and diffuses the rayssufficiently to impart a bright white appearance to reflective surface190. Most light rays which pass through microlens array 180 to reachsurface 190 within about 30° of the normal direction are reflected bysurface 190 back toward microlens array 180 within the same angularrange, as shown in FIG. 6A. In a reflective display application theincident light rays typically fall within about a 30° angular range ofthe normal direction. The embodiment of FIGS. 6A and 6B enhances neteffective reflectance by a factor of about 2 relative to that of ahighly reflective diffuse material. The magnitude of the enhancementfactor depends on the geometry of microlens array 180, its distance fromsurface 190 and the reflectance characteristics of surface 190.

FIG. 7 depicts a semi-diffuse and semi-retro-reflective reflector 220having a wide effective angular viewing range. Reflector 220incorporates an array 222 of microlenses 224, 226, 228; and a reflectivesurface 230 positioned near the effective focal points 231 of therespective microlenses 224, 226, 228. Microlenses 224, 226, 228 arepositioned in front of (i.e. on an outward side of) reflective surface230 and protrude inwardly toward surface 230, which may be diffusely orspecularly reflective. Incident light rays (i.e. light rays which arrivefrom approximately the typical viewing direction) such as rays 232, 234,236 which undergo TIR within microlenses 224, 226, 228 respectively aresemi-retro-reflected back through microlenses 224, 226, 228 respectivelyas previously described in relation to rays 68, 72, 74, 76 depicted inFIG. 2; whereas light rays which pass through microlens array 222—suchas rays 238, 240, 242—and which reach reflective surface 230 arereflected by surface 230 from near one of focal points 231 and hence arealso semi-retro-reflected back through microlens array 222. Thus, almostall of the incident light rays are reflected toward the viewer, withinthe desired angular range.

The degree of sharpness or clarity of retro-reflection attained byreflector 220 can be modified by adjusting the distance d₂ betweenmicrolens array 222 and reflective surface 230. Generally, if the degreeof retro-reflection is too sharp, the display's luminance is reduced dueto the absence of a source of light rays collinear with the viewingdirection—the viewer's head will obstruct such rays. Recall thatreflective (i.e. front-lit) displays rely upon an external light sourcewhich may be obstructed by the viewer's head in some cases. However, ifthe degree of retro-reflection is insufficiently sharp, inadequateretro-reflectivity is attained, so an optimal intermediate degree ofretro-reflection should be selected. Generally, it is desirable for theperiodicity of any microstructures incorporated in reflective surface230 to be large compared to the periodicity of microlens array 222, butthis is not always necessary.

FIGS. 8A and 8B schematically depict electrophoretic modulation of theembodiment of FIGS. 6A and 6B, with FIG. 8A depicting the reflectivestate and FIG. 8B depicting the absorptive, non-reflective state. An airor liquid electrophoresis medium 204 is contained between microlensarray 180 and reflective surface 190. Light absorbing particles (e.g.pigment particles—not shown) are suspended in electrophoresis medium204. A voltage is applied across electrophoresis medium 204 aspreviously described in relation to FIG. 1A, with annular patternsegments 192 constituting the backplane electrode. In the reflectivestate (FIG. 8A) the particles are attracted to and clumped aroundannular pattern segments 192. This leaves circular, reflective portions194 of surface 190 substantially unobstructed by the particles, thuspermitting reflection of light rays by the circular, reflective portions194 of surface 190 as previously explained in relation to FIG. 6A. Inthe absorptive, non-reflective state (FIG. 8B) the particles areattracted to and are distributed more or less uniformly across theinward surface 206 of microlens array 180 (which bears a transparentelectrode—not shown). Light rays which pass through microlens array 180are absorbed by the particles at the inward surface 206 of microlensarray 180.

FIGS. 9A and 9B schematically depict modulation of the embodiment ofFIGS. 6A and 6B by electro-deformation of a liquid medium 208 such asoil containing a light absorptive dye or dye mixture. FIG. 9A depictsthe electro-deformed, reflective state. FIG. 9B depicts the relaxed,absorptive (non-reflective) state. Liquid electro-deformation isdescribed by Aggarwal et al. in “Liquid Transport Based on ElectrostaticDeformation of Fluid Interfaces” Journal of Applied Physics 99, 104904published online 25 May 2006.

A voltage is applied across liquid medium 208 as previously described inrelation to FIG. 1A, with the backplane electrode conforming to theshape of annular pattern segments 192. Liquid medium 208 always remainson reflective surface 190—liquid medium 208 does not cross the gapbetween microlens array 180 and reflective surface 190. In theelectro-deformed, reflective state (FIG. 9A) liquid medium 208 is movedaway from the circular, reflective portions 194 of surface 190 beneaththe respective microlenses in array 180, and is redistributed inapproximately hemi-toroidal shapes atop each of annular pattern segments192 as seen in FIG. 9A. This leaves circular, reflective portions 194 ofsurface 190 substantially unobstructed by absorptive liquid medium 208,thus permitting reflection of light rays by the circular, reflectiveportions 194 of surface 190 as previously explained in relation to FIGS.6A and 6B. In the relaxed, absorptive (non-reflective) state (FIG. 9B)liquid medium 208 is distributed more or less uniformly acrossreflective surface 190, obstructing substantially all reflectiveportions 194 of surface 190. Light rays which are converged throughmicrolens array 180 toward surface 190 are thus absorbed wherever theyencounter surface 190.

FIGS. 10A and 10B schematically depict electrophoretic modulation of theFIG. 7 embodiment, with FIG. 10A depicting the reflective state and FIG.10B depicting the absorptive, non-reflective state. An air or liquidelectrophoresis medium 250 is contained between transparent sheets 252,254 on the outward side of microlens array 222. Light absorbingparticles (e.g. pigment particles—not shown) are suspended inelectrophoresis medium 250. A voltage is applied across electrophoresismedium 250 via transparent electrodes on the internally opposed sides ofsheets 252, 254. The electrode on inward sheet 254 may conform to theshape of microlens array 222's reflective annular regions (analogous tohemi-bead 60's reflective, annular region 80 shown in FIGS. 4A-4G) ormay be formed as an array of thin lines, etc.

In the reflective state (FIG. 10A) the particles are attracted to andclumped around the electrode segments on inward sheet 254, as indicatedat 256. This leaves circular regions on inward sheet 254 substantiallyunobstructed by the particles, thus permitting semi-retro-reflection oflight rays which are transmitted through microlens array 222 andreflected by surface 230 back through microlens array 222. Some lightrays (e.g. those labelled 258 and 260 in FIG. 10A) are lost due toabsorption, so it is desirable to minimize the area occupied by theelectrode segments on inward sheet 254.

In the absorptive, non-reflective state (FIG. 10B) the particles areattracted to and are distributed more or less uniformly across theinward surface of sheet 252, as indicated at 262. Light rays which passthrough sheet 252 (i.e. substantially all incident light rays)) areabsorbed by the particles at the inward surface of sheet 252 as shown inFIG. 10B.

FIGS. 11A and 11B schematically depict modulation of the FIG. 7embodiment by electro-deformation of a liquid medium 270 such as oilcontaining a light absorptive dye or dye mixture. Liquid-medium 270 iscontained between transparent sheets 252, 254 on the outward side ofmicrolens array 222. FIG. 11A depicts the electro-deformed, reflectivestate. FIG. 11B depicts the relaxed, absorptive (non-reflective) state.A voltage is applied across liquid medium 270 via transparent electrodeson the internally opposed sides of sheets 252, 254 (similar to theelectrodes described above in relation to FIGS. 10A and 10B). In theelectro-deformed, reflective state (FIG. 11A) liquid medium 270 isredistributed in approximately hemi-toroidal shapes 272 atop theelectrode segments on inward sheet 254. This leaves circular regions oninward sheet 254 substantially unobstructed, thuspermitting-semi-retro-reflection of light rays which are transmittedthrough microlens array 222 and reflected by surface 230 back throughmicrolens array 222. Some light rays (e.g. those labelled 258 and 260 inFIG. 11A) are lost due to absorption, so it is desirable to minimize thearea occupied by the electrode segments on inward sheet 254.

In the relaxed, absorptive (non-reflective) state (FIG. 11B) liquidmedium 270 is distributed more or less uniformly across the inwardsurface of sheet 252, as indicated at 274. Light rays which pass throughsheet 252 (i.e. substantially all incident light rays)) are absorbed byliquid medium 270 at the inward surface of sheet 252 as shown in FIG.11B.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. All suchmodifications, permutations, additions and sub-combinations are withinthe true spirit and scope of the invention.

1. A reflective display, comprising a rearward light reflector the lightreflector further comprising a microlens array.
 2. A reflective displayas defined in claim 1, wherein the microlens array further comprises aplurality of hemispherical microlenses.
 3. A reflective display asdefined in claim 1, wherein the microlens array further comprises aplurality of approximately hemispherical microlenses.
 4. A reflectivedisplay as defined in claim 1, further comprising: a reflective surfacespaced inwardly from the microlens array; an electrode on the reflectivesurface, the electrode having a plurality of annular segments, eachannular segment aligned with one of the microlenses; a light absorptiveliquid medium on the reflective surface; an electrical potential sourceelectrically connected to apply an electrical potential across theliquid medium; and wherein the liquid medium is oil.
 5. A reflectivedisplay as defined in claim 1, further comprising: a reflective surfacespaced inwardly from the microlens array; an electrode on the reflectivesurface, the electrode having a plurality of annular segments, eachannular segment aligned with one of the microlenses; an electrophoresismedium contained between the microlens array and the reflective surface;a plurality of light absorptive particles suspended in theelectrophoresis medium; and an electrical potential source electricallyconnected to apply an electrical potential across the electrophoresismedium.
 6. A reflective display as defined in claim 5, wherein theelectrophoresis medium is a liquid.
 7. A reflective display as definedin claim 5, wherein the electrophoresis medium is air.
 8. A reflectivedisplay as defined in claim 1, wherein: the electrophoresis medium is aliquid; the liquid medium is oil; the reflector further comprises aspecular reflector; and the specular reflector is spaced inwardly fromthe microlens array such that a substantial fraction of light raysrefracted through the microlens array toward the specular reflector areapproximately retro-reflected by the specular reflector through themicrolens array.
 9. A reflective display as defined in claim 3, furthercomprising: a reflective surface spaced inwardly from the microlensarray; a light absorptive liquid medium contained on an outward side ofthe microlens array between outward and inward transparent sheets; anelectrode on the inward sheet, the electrode having a plurality ofsegments, each segment aligned with one of the microlenses; and anelectrical potential source electrically connected to apply anelectrical potential across the liquid medium.
 10. A reflective displayas defined in claim 9, wherein the liquid medium is oil.
 11. Areflective display as defined in claim 5, wherein: the reflector furthercomprises a specular reflector; and the specular reflector is spacedinwardly from the microlens array such that a substantial fraction oflight rays refracted through the microlens array toward the specularreflector are approximately retro-reflected by the specular reflectorthrough the microlens array.
 12. A reflective display as defined inclaim 1, wherein: the microlens array further comprises a plurality ofmicrolenses shaped such that a substantial fraction of light raysincident on the microlens array are redirected onto a reflective regionbeneath the microlenses; the reflector further comprises a specularreflector; and the specular reflector is spaced inwardly from themicrolens array such that a substantial fraction of light rays refractedthrough the microlens array toward the specular reflector areapproximately retro-reflected by the specular reflector through themicrolens array.
 13. A reflective display as defined in claim 11,wherein the microlens array further comprises a plurality ofapproximately hemispherical microlenses.
 14. A reflective display asdefined in claim 2, further comprising: for substantially each one ofthe microlenses, a reflective region aligned with the one of themicrolenses, and wherein substantially each one of the microlenses isshaped such that a substantial fraction of light rays incident on theone of the microlenses are redirected onto the reflective region alignedwith the one of the microlenses.
 15. A reflective display as defined inclaim 2, further comprising: a reflective surface spaced inwardly fromthe microlens array; an electrophoresis medium contained on an outwardside of the microlens array between outward and inward transparentsheets; an electrode on the inward sheet, the electrode having aplurality of segments, each segment aligned with one of the microlenses;a plurality of light absorptive particles suspended in theelectrophoresis medium; an electrical potential source electricallyconnected to apply an electrical potential across the electrophoresismedium; the reflector further comprises a specular reflector; and thespecular reflector is spaced inwardly from the microlens array such thata substantial fraction of light rays refracted through the microlensarray toward the specular reflector are approximately retro-reflected bythe specular reflector through the microlens array.
 16. A reflectivedisplay as defined in claim 1, further comprising: for substantiallyeach one of the microlenses, a reflective region aligned with the one ofthe microlenses, and wherein substantially each one of the microlensesis shaped such that a substantial fraction of light rays incident on theone of the microlenses are redirected onto the reflective region alignedwith the one of the microlenses; a reflective surface spaced inwardlyfrom the microlens array; a light absorptive liquid medium contained onan outward side of the microlens array between outward and inwardtransparent sheets; an electrode on the inward sheet, the electrodehaving a plurality of segments, each segment aligned with one of themicrolenses; an electrical potential source electrically connected toapply an electrical potential across the liquid medium; and wherein theliquid medium is oil.
 17. A reflective display as defined in claim 15,the microlens array further compriseing a plurality of microlensesshaped such that a substantial fraction of light rays incident on themicrolens array are redirected onto a reflective region beneath themicrolenses.
 18. A reflective display as defined in claim 2, themicrolens array further comprising a plurality of microlenses shapedsuch that a substantial fraction of light rays incident on the microlensarray are redirected onto a reflective region beneath the microlenses;further comprising, for substantially each one of the microlenses, areflective region aligned with the one of the microlenses, and whereinsubstantially each one of the microlenses is shaped such that asubstantial fraction of light rays incident on the one of themicrolenses are redirected onto the reflective region aligned with theone of the microlenses; further comprising: a reflective surface spacedinwardly from the microlens array; a light absorptive liquid mediumcontained on an outward side of the microlens array between outward andinward transparent sheets; an electrode on the inward sheet, theelectrode having a plurality of segments, each segment aligned with oneof the microlenses; and an electrical potential source electricallyconnected to apply an electrical potential across the liquid medium. 19.A reflective display as defined in claim 2, the microlens array furthercomprising a plurality of microlenses shaped such that a substantialfraction of light rays incident on the microlens array are redirectedonto a reflective region beneath the microlenses; further comprising,for substantially each one of the microlenses, a reflective regionaligned with the one of the microlenses, and wherein substantially eachone of the microlenses is shaped such that a substantial fraction oflight rays incident on the one of the microlenses are redirected ontothe reflective region aligned with the one of the microlenses; furthercomprising: a reflective surface spaced inwardly from the microlensarray; an electrophoresis medium contained on an outward side of themicrolens array between outward and inward transparent sheets; anelectrode on the inward sheet, the electrode having a plurality ofsegments, each segment aligned with one of the microlenses; a pluralityof light absorptive particles suspended in the electrophoresis medium;and an electrical potential source electrically connected to apply anelectrical potential across the electrophoresis medium.
 20. A reflectivedisplay as defined in claim 19, wherein the electrophoresis medium isair.